I have a console app and I want to launch tasks one after the other.
Here is my code:
static void Main()
{
string keywords = "Driving Schools,wedding services";
List<string> kwl = keywords.Split(',').ToList();
foreach(var kw in kwl)
{
Output("SEARCHING FOR: " + kw);
Task t = new Task(() => Search(kw));
t.Start();
t.Wait();
}
Console.ReadLine();
}
static async void Search(string keyword)
{
// code for searching
}
The problem is that it doesn't wait for the first task to finish executing. It fires off the subsequent tasks concurrently.
I am working with a rate limited API so I want to do one after the other.
Why is it not waiting for one search to finish before starting the next search?
Your async method just returns void, which means there's no simple way of anything waiting for it to complete. (You should almost always avoid using async void methods. They're really only available for the sake of subscribing to events.) Your task just calls Search, and you're waiting for that "I've called the method" to complete... which it will pretty much immediately.
It's not clear why you're using async at all if you actually want to do things serially, but I'd suggest changing your code to look more like this:
static void Main()
{
// No risk of deadlock, as a console app doesn't have a synchronization context
RunSearches().Wait();
Console.ReadLine();
}
static async Task RunSearches()
{
string keywords = "Driving Schools,wedding services";
List<string> kwl = keywords.Split(',').ToList();
foreach(var kw in kwl)
{
Output("SEARCHING FOR: " + kw);
await Search(kw);
}
}
static async Task Search(string keyword)
{
// code for searching
}
Related
ok. I made a simple console app to figure out how to make all this work. Once I have the basic outline working, then I'll apply it to the real application.
The idea is that we have a lot of database calls to execute that we know are going to take a long time. We do NOT want to (or have to) wait for one database call to be completed before we make the next. They can all run at the same time.
But, before making all of the calls, we need to perform a "starting" task. And when all of the calls are complete, we need to perform a "finished" task.
Here's where I'm at now:
static void Main(string[] args)
{
Console.WriteLine("starting");
PrintAsync().Wait();
Console.WriteLine("ending"); // Must not fire until all tasks are finished
Console.Read();
}
// Missing an "await", I know. But what do I await for?
static async Task PrintAsync()
{
Task.Run(() => PrintOne());
Task.Run(() => PrintTwo());
}
static void PrintOne()
{
Console.WriteLine("one - start");
Thread.Sleep(3000);
Console.WriteLine("one - finish");
}
static void PrintTwo()
{
Console.WriteLine("two - start");
Thread.Sleep(3000);
Console.WriteLine("two - finish");
}
But no matter what I try, Ending always gets printed too early:
starting
ending
one - start
two - start
one - finish
two - finish
What IS working right is that PrintTwo() starts before PrintOne() is done. But how do I properly wait for PrintAsync() to finish before doing anything else?
you need to await the ending of the inner tasks:
static async Task PrintAsync()
{
await Task.WhenAll(Task.Run(() => PrintOne()), Task.Run(() => PrintTwo()));
}
explanation: async Task denotes an awaitable method. Inside this method you can also await Tasks. If you don't do this then it will simply let the tasks loose which will run on their own. Task.Run returns a Task which can be awaited. If you want both tasks to run in parallel you can use the tasks from the retur values and use them in the awaitable method Task.WhenAll
EDIT: Actually Visual Studio would mark this code with a green curvy line. When hoovering with the mouse over it you get a warning:
CS4014
This should explain why "ending" is printed before the tasks have finished
EDIT 2:
If you have a collection of parameters that you want to iterate and call an async method to pass the parameter in, you can also do it with a select statement in 1 line:
static async Task DatabaseCallsAsync()
{
// List of input parameters
List<int> inputParameters = new List<int> {1,2,3,4,5};
await Task.WhenAll(inputParameters.Select(x => DatabaseCallAsync($"Task {x}")));
}
static async Task DatabaseCallAsync(string taskName)
{
Console.WriteLine($"{taskName}: start");
await Task.Delay(3000);
Console.WriteLine($"{taskName}: finish");
}
The last part is similar to a previous answer
OP here. I'm going to leave the answer by Mong Zhu marked as correct, as it lead me to the solution. But I also want to share the final result here, which includes excellent feedback in the comments from juharr. Here's what I came up with:
class Program
{
static void Main(string[] args)
{
Console.WriteLine("starting");
DatabaseCallsAsync().Wait();
Console.WriteLine("ending"); // Must not fire until all database calls are complete.
Console.Read();
}
static async Task DatabaseCallsAsync()
{
// This is one way to do it...
var tasks = new List<Task>();
for (int i = 0; i < 3; i++)
{
tasks.Add(DatabaseCallAsync($"Task {i}"));
}
await Task.WhenAll(tasks.ToArray());
// This is another. Same result...
List<int> inputParameters = new List<int> { 1, 2, 3, 4, 5 };
await Task.WhenAll(inputParameters.Select(x => DatabaseCallAsync($"Task {x}")));
}
static async Task DatabaseCallAsync(string taskName)
{
Console.WriteLine($"{taskName}: start");
await Task.Delay(3000);
Console.WriteLine($"{taskName}: finish");
}
}
Here's the result:
starting
Task 0: start
Task 1: start
Task 2: start
Task 2: finish
Task 0: finish
Task 1: finish
ending
I have four methods.
Main: only calls the preform method
Working: displays "please wait for the user"
Taking Time: A program that takes time to execute.
Preform: Calls the taking time and working methods asynchronously.
The following is my code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
namespace AsyncObservation
{
class Program
{
static void Main(string[] args)
{
preform();
}
public static async Task Working()
{
Console.WriteLine("Please wait, the program is running");
}
public static async Task Takingtime()
{
Console.WriteLine("This Program started");
Thread.Sleep(1000);
Console.WriteLine("The Program finished");
}
public static async void preform()
{
Task timer = Takingtime();
Task wait = Working();
}
}
}
In the end: I need to display
This program started.
Please wait, the program is running
The program ended.
I see several problems in your program.
Although Preform is neither async nor an event handler, it does not return a Task
The tasks started in Preform are not awaited for before you finish Preform. Hence you never know when they are finished, nor what the results are (exception?). You could even end your program before they are finished
After you start a Task it is not guaranteed when it will run. You can only be sure that statements are already executed if you await for the Task.
Using async-await is a method to make sure that your thread looks around to see if it can do useful stuff instead of waiting idly if it has to wait for something. Thread.Sleep is a busy wait. If you want to look around to see if you can do something else use await Task.Delay(TimeSpan.FromSeconds(1)) instead.
In your case, you can't be sure that any Console line has been written until you await the procedure that should write your line. If you start your second Task before awaiting the first, you don't know how far the first task already proceeded, and thus you don't know for sure that the text already has been written to the Console.
C# 7.1 introduced async Task Main(), so you could use that instead of the traditional void Main. It saves you from catching and interpreting the AggregateException that is thrown by the Task you start to make your process async.
If you don't want to use the async Main, you can of course use Task.Run to call an async function:
static void Main(string[] args)
{
try
{
var preformTask = Task.Run( () => Preform() );
DoSomethingElse(); // if needed
preformTask.Wait(); // wait for preformTask to finish
Console.WriteLine("Task completed; press any key to finish");
Console.ReadKey();
}
catch (Exception exc) // inclusive ggregateException if one of your Task fails
{
ProcessException(exc)
}
}
static async Task preform()
{
// To be certain that the Console Line has been written: await
await Takingtime();
// if here, you are certain that the Line has been written,
// or course you have lost parallel processing
await Working();
}
For completeness: the other functions
public static async Task Working()
{
Console.WriteLine("Please wait, the program is running");
// either return a completed Task, or await for it (there is a difference!
await Task.CompletedTask;
// or:
return Task.CompletedTask; // do not declare async in this case
}
public static async Task Takingtime()
{
Console.WriteLine("This Program started");
//Use Task.Delay instead of Sleep
await Task.Delay(TimeSpan.FromSeconds(1); // improved readability
Console.WriteLine("The Program finished");
}
Because of the awaits in Preform you are certain that the text has been written. However, you've lost some parallellism.
If you really want those procedures to execute at the same time, you can't be certain about when text will be written. If that is important, then split the Parts that should be run first (write Console) from the parts that should run in parallel (Task.Delay)
static async Task preform()
{
// Do the things that should be done before parallel tasks are run
await DoThisFirst();
// start the Tasks that can work parallel: not sure what statements are executed first
var taskA = DoTaskA();
var taskB = DoTaskB();
// if here, you are free to do something else
// can't be sure about the status of taskA nor taskB
DoSomethingElse();
// if you want to do something after you know that the tasks have completed:
// await the tasks here:
await Task.When (new Task[] {taskA, taskB});
// if here, you are certain that all parallel tasks have completed successfully
// if desired fetch the return values of the Tasks:
var returnValueA = taskA.Result;
var returnValueB = taskB.Result;
// do all async things of which you needed to be certain that both tasks finished
// for example:
await ProcessResults(returnValueA, returnValueB);
}
In a console app it is OK to use a .Wait() call in an void Main method.
In some contexts where there is synchronisation necessary .Wait() can cause deadlocks (ASP.NET has a request to synchronise on or XAML/WinForms which have a UI thread), but there is nothing to synchronise on here.
static void Main()
{
preform.Wait()
}
This will wait for the async work to complete synchronously. You need the method to run synchronously so that it does not return early. If you use async void and await the method will return immediately and exit the method, await does not work in a Main method.
I would also suggest using await Task.Delay(1000); rather than Thread.Sleep(1000); as this is the canonical async way to idle.
For your code example:
class Program
{
static void Main(string[] args)
{
preform().Wait();
Console.ReadLine();
}
public static async Task Working()
{
Console.WriteLine("Please wait, the program is running");
}
public static async Task Takingtime()
{
Console.WriteLine("This Program started");
await Task.Delay(1000);
Console.WriteLine("The Program finished");
}
public static Task preform()
{
return Task.WhenAll(
Takingtime(),
Working());
}
}
Use Stephen Cleary's Nito.AsyncEx library (available via Nuget) to provide an asynchronous context for a console application. See here for more details.
Your application can then be written as ...
class Program
{
static int Main(string[] args)
{
try
{
Console.WriteLine("The application has started");
AsyncContext.Run(() => LongRunningTaskAsync(args));
Console.WriteLine("The application has finished");
}
catch (Exception ex)
{
Console.Error.WriteLine(ex);
return -1;
}
}
static async Task LongRunningTaskAsync(string[] args)
{
Console.WriteLine("The long running task has started");
// use Task.Delay() rather than Thread.Sleep() to avoid blocking the application
await Task.Delay(TimeSpan.FromSeconds(10)).ConfigureAwait(false);
Console.WriteLine("The long running task has finished");
}
}
Use preform().Wait() in the constructor. So the call is awaited. Then use await Takingtime() and await Working() in the preform method.
And you have to change the return type to Task.
I'm trying to find out how to use WhenAll to let two methods run at once, and once they both finish, collect the results without blocking by using .Result
I have this little console app test:
using System.Diagnostics;
using System.Threading.Tasks;
namespace ConsoleApplication2
{
class Program
{
public static void Main(string[] args)
{
var run = TaskRunner();
Debug.WriteLine(run);
if (run.IsCompleted)
{
Debug.WriteLine("this worked!");
} else
{
Debug.WriteLine("this failed!");
}
}
public static async Task<string> TaskRunner()
{
var taskOne = OneAsync();
var taskTwo = TwoAsync();
var tasks = await Task.WhenAll(taskOne, taskTwo);
var retval = tasks[0] + tasks[1];
return retval;
}
public static Task<string> OneAsync()
{
return Task.Run(() =>
{
return "test1";
});
}
public static Task<string> TwoAsync()
{
return Task.Run(() =>
{
return "test2";
});
}
}
}
This currently prints this worked! to my Output window... However, if I comment out Debug.WriteLine(run); it prints this failed!... Why does the Task complete simply by being logged to the output window?
I'm trying to understand a huge problem in a complex piece of code and this little test is my MCVE to hopefully shed some light on what is happening behind the scenes.
This happens just by pure chance. The way you are starting your task is with Task.Run. This essentially creates a new thread on which the (synchronous) action is executed. It returns a task for the completion of that thread.
So OneAsync and TwoAsync will each spawn a new thread that then immediately returns a string. This will happen very quickly but there’s still some overhead for creating those threads which means that it won’t be instantaneous.
TaskRunner then calls both those methods (spawning the threads), and then asynchronously waits for both threads to finish. Since the threads are not completely instantly, this TaskRunner method also won’t complete instantly.
Now, in your main, you are starting the asynchronous TaskRunner, which we figured will take “a very short moment”. You do not await the task, so the execution continues immediately. Debug.WriteLine is executed to print something (it probably doesn’t really matter that it’s the task in question that is being printed), and then you are checking the state of the task.
Since printing stuff is relatively slow (compared to other operations), this is probably the reason why the tasks ends up being completed. And when you remove the printing, the if is just reached too quickly for the task to finish.
As you likely noticed, working like that with asynchronous tasks does not appear to be a good idea. That’s why you should always await the task when you depend on its result.
// note the *async* here; async main methods are supported with C# 7.1
public static async void Main(string[] args)
{
var run = TaskRunner();
// await the task
await run;
if (run.IsCompleted)
{
Debug.WriteLine("this worked!");
}
else
{
Debug.WriteLine("this failed!");
}
}
For the past several days, I've been trying to figure out why my async method is not working and its a very simple piece of code.
public class EntryPoint
{
static void Main()
{
RunTheTaskAsync();
//Console.ReadLine(); // if I add this to "hold" the finish of the project, I will see the result, without it I dont
}
public async static void RunTheTaskAsync()
{
Task<string> task = Concatenator('1', 200000);
Console.WriteLine("Heeeelllooooooo");
Console.WriteLine("I am running while Concatenator is concatenating in the background.");
Console.WriteLine("You will receive the results shortly");
string result = await task;
Console.WriteLine("Result is: " + result.Length);
}
public static Task<string> Concatenator(char characterToConcatenate, int count)
{
Console.WriteLine("Concatenating!");
return Task<string>.Factory.StartNew(() =>
{
string concatenatedString = "";
for (int i = 0; i < count; i++)
{
concatenatedString += characterToConcatenate;
}
return concatenatedString;
});
}
}
If I run it as it is in the example above, I never get to see the result, I only see the Console.WriteLine's and Press any key to continue.. after that no result.
If I uncomment the Console.ReadLine(); everything works as I am expecting it to work and I simply cant understand why!?! The same is true if I add some Thread.Sleep(xxx) or another piece of code that will take longer to execute than the "Concatenator" method.
Isn't this one of the issues that await should solve on its own, its kind of implied from its very name.
The way I understand it is:
The Task method starts executing on the background
We proceed with the 3 ConsoleWritelines in the async method
Since there is nothing else to do and the task is not completed yet, we get to the await line and we should await for the task
to be completed and then proceed with the rest of the code.
It works like this ONLY if have another piece of code inbetween that will take longer than the Task method to execute, or if I use the Console.ReadLine() and I just cant find an explanation about this!
I also have a secondary question regarding the syntax of my implementation. Is this the proper way to create a Task method? By using
public Task<T> SomeMethodName(some, args)
{
return Task<T>Factory.StartNew(() =>
{
somecode that returns T;
});
}
async voids (like your RunTheTaskAsync method) are "fire and forget". You can't track their progress and almost never want to use them.
Instead, make an async Task and await it:
static async Task Main()
{
await RunTheTaskAsync();
}
public static async Task RunTheTaskAsync()
{
...
}
For more details, see:
Async/Await - Best Practices in Asynchronous Programming
Note: Visual Studio prior to 2017 Update 3 does not support an async Main method. In that case, you have to manually await the Task:
static void Main()
{
RunTheTaskAsync().GetAwaiter().Wait();
}
About your second question (for the future: SO's policy is "one question per question"): Yes, Task.Factory.StartNew is correct if you need your task to run in a separate thread. Newer versions of .NET as offer Task.Run as a shortcut, see the following question for details:
What is the difference between Task.Run() and Task.Factory.StartNew()
I am rather new to task based programming and trying to determine how to return a task and verify that it has been started. The code that I got to work was not what I was expecting. The console application is as follows:
public static void Main(string[] args)
{
var mySimple = new Simple();
var cts = new CancellationTokenSource();
var task = mySimple.RunSomethingAsync(cts.Token);
while (task.Status != TaskStatus.RanToCompletion)
{
Console.WriteLine("Starting...");
Thread.Sleep(100);
}
Console.WriteLine("It is started");
Console.ReadKey();
cts.Cancel();
}
public class Simple
{
public async void RunSomething(CancellationToken token)
{
var count = 0;
while (true)
{
if (token.IsCancellationRequested)
{
break;
}
Console.WriteLine(count++);
await Task.Delay(TimeSpan.FromMilliseconds(1000), token).ContinueWith(task => { });
}
}
public Task RunSomethingAsync(CancellationToken token)
{
return Task.Run(() => this.RunSomething(token));
}
}
The output is:
Starting...
0
It is started
1
2
3
4
Why is the task that is being returned have a status as TaskStatus.RanToCompletion compared to TaskStatus.Running as we see that the while loop is still executing? Am I checking the status of the task of putting the RunSomething task on the threadpool rather than the RunSomething task itself?
RunSomething is an async void method, meaning it exposes no means of the caller ever determining when it finishes, they can only ever start the operation and then have no idea what happens next. You then wrap a call to it inside of Task.Run, this is schedluing a thread pool thread to start RunSomething. It will then complete as soon as it has finished starting that Task.
If RunSomething actually returned a Task, then the caller would be able to determine when it actually finished, and if you waited on it it wouldn't actually indicate that it was done until that asynchronous operation was actually finished (there would be no reason to use Task.Run to start it in another thead, you'd be better off just calling it directly and not wasting the effort of moving that to a thread pool thread).
Never use async void (https://msdn.microsoft.com/en-us/magazine/jj991977.aspx)
instead you should use async Task
If you need to call an async method from a non-async (such as from a static void main) you should do something like this:
mySimple.RunSomethingAsync(cts.Token).GetAwaiter().GetResult();
That will effectively make the method a synchronous call.
You can use async void, but only for events.